Yet it says he was not an only child. Were the others immaculately conceived too? Why is it so important to some to "prove" she remained a virgin when the real story is the initial virgin birth?
By what tenet/canon is she required to have remained a virgin after bringing Jesus into the world? It might make some feel better to envision her as an eternal virgin, but would God do that to Joseph after hitting him with the initial surprise? God meant for husbands and wives to unite in order to procreate - why is it so hard to think of Mary as a good wife after her initial task was done?
Because the historical-critical method is lacking in understanding what has taken place. Mary is the Ark of the New Covenant. Consecrated to God as the new Tabernacle and Temple of the Lord.
“Yet it says he was not an only child.”
No, not at all. And why would that matter to you (since you appear to reject the teaching authority of the Church)?
Because some Catholics are hung up on the sex act itself.